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RFC 1738 - 5. BNF for specific URL schemes

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### 5. BNF for specific URL schemes

This is a BNF-like description of the Uniform Resource Locator syntax, using the conventions of RFC822, except that "|" is used to designate alternatives, and brackets [] are used around optional or repeated elements. Briefly, literals are quoted with "", optional elements are enclosed in [brackets], and elements may be preceded with <n>* to designate n or more repetitions of the following element; n defaults to 0.

    ; The generic form of a URL is:

    genericurl     = scheme ":" schemepart

    ; Specific predefined schemes are defined here; new schemes
    ; may be registered with IANA

    url            = httpurl | ftpurl | newsurl |
                     nntpurl | telneturl | gopherurl |
                     waisurl | mailtourl | fileurl |
                     prosperourl | otherurl

    ; new schemes follow the general syntax
    otherurl       = genericurl

    ; the scheme is in lower case; interpreters should use case-ignore
    scheme         = 1*[ lowalpha | digit | "+" | "-" | "." ]
    schemepart     = *xchar | ip-schemepart

    ; URL schemeparts for ip based protocols:

    ip-schemepart  = "//" login [ "/" urlpath ]

    login          = [ user [ ":" password ] "@" ] hostport
    hostport       = host [ ":" port ]
    host           = hostname | hostnumber
    hostname       = *[ domainlabel "." ] toplabel
    domainlabel    = alphadigit | alphadigit *[ alphadigit | "-" ] alphadigit
    toplabel       = alpha | alpha *[ alphadigit | "-" ] alphadigit
    alphadigit     = alpha | digit
    hostnumber     = digits "." digits "." digits "." digits
    port           = digits
    user           = *[ uchar | ";" | "?" | "&" | "=" ]
    password       = *[ uchar | ";" | "?" | "&" | "=" ]
    urlpath        = *xchar    ; depends on protocol see section 3.1

    ; The predefined schemes:

    ; FTP (see also RFC959)

    ftpurl         = "ftp://" login [ "/" fpath [ ";type=" ftptype ]]
    fpath          = fsegment *[ "/" fsegment ]
    fsegment       = *[ uchar | "?" | ":" | "@" | "&" | "=" ]
    ftptype        = "A" | "I" | "D" | "a" | "i" | "d"

    ; FILE

    fileurl        = "file://" [ host | "localhost" ] "/" fpath

    ; HTTP

    httpurl        = "http://" hostport [ "/" hpath [ "?" search ]]
    hpath          = hsegment *[ "/" hsegment ]
    hsegment       = *[ uchar | ";" | ":" | "@" | "&" | "=" ]
    search         = *[ uchar | ";" | ":" | "@" | "&" | "=" ]

    ; GOPHER (see also RFC1436)

    gopherurl      = "gopher://" hostport [ / [ gtype [ selector
                     [ "%09" search [ "%09" gopher+_string ] ] ] ] ]
    gtype          = xchar
    selector       = *xchar
    gopher+_string = *xchar

    ; MAILTO (see also RFC822)

    mailtourl      = "mailto:" encoded822addr
    encoded822addr = 1*xchar               ; further defined in RFC822

    ; NEWS (see also RFC1036)

    newsurl        = "news:" grouppart
    grouppart      = "*" | group | article
    group          = alpha *[ alpha | digit | "-" | "." | "+" | "_" ]
    article        = 1*[ uchar | ";" | "/" | "?" | ":" | "&" | "=" ] "@" host

    ; NNTP (see also RFC977)

    nntpurl        = "nntp://" hostport "/" group [ "/" digits ]

    ; TELNET

    telneturl      = "telnet://" login [ "/" ]

    ; WAIS (see also RFC1625)

    waisurl        = waisdatabase | waisindex | waisdoc
    waisdatabase   = "wais://" hostport "/" database
    waisindex      = "wais://" hostport "/" database "?" search
    waisdoc        = "wais://" hostport "/" database "/" wtype "/" wpath
    database       = *uchar
    wtype          = *uchar
    wpath          = *uchar

    ; PROSPERO

    prosperourl    = "prospero://" hostport "/" ppath *[ fieldspec ]
    ppath          = psegment *[ "/" psegment ]
    psegment       = *[ uchar | "?" | ":" | "@" | "&" | "=" ]
    fieldspec      = ";" fieldname "=" fieldvalue
    fieldname      = *[ uchar | "?" | ":" | "@" | "&" ]
    fieldvalue     = *[ uchar | "?" | ":" | "@" | "&" ]

    ; Miscellaneous definitions

    lowalpha       = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                     "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                     "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                     "y" | "z"
    hialpha        = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
                     "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
                     "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
    alpha          = lowalpha | hialpha
    digit          = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                     "8" | "9"
    safe           = "$" | "-" | "_" | "." | "+"
    extra          = "!" | "*" | "'" | "(" | ")" | ","
    national       = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`"
    punctuation    = "<" | ">" | "#" | "%" | <">

    reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "="
    hex            = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                     "a" | "b" | "c" | "d" | "e" | "f"
    escape         = "%" hex hex

    unreserved     = alpha | digit | safe | extra
    uchar          = unreserved | escape
    xchar          = unreserved | reserved | escape
    digits         = 1*digit

Next: 6. Security Considerations

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RFC 1738 - 5. BNF for specific URL schemes